Studies on mechanisms of action of hormones are performed in the least ambiguous fashion when the environment of the target cell can be closely controlled, and when cause-and-effect (not simply correlative) relationships can be established. Previous work in this laboratory on hamster yolk-sac erythroid cells has shown that these cells: undergo nearly the entire spectrum of globin ontogeny in the embryonic circulation; demonstrate globin ontogeny when grown in culture if both erythropoietin and triiodothyronine are present in the culture medium; and possess plasma-membrane receptors specific for erythropoietin and nuclear receptors specific for triiodothyronine. We now propose experiments designed to investigate responses of hamster yolk-sac erythroid cells to these hormones, singly and together. The experimental approach will include attempts to isolate the hormone-specific receptors and attempts to quantify receptor- specific mRNA.